射频加热技术在食品工业中的应用

射频加热技术作为一种新型热加工技术,其在食品干燥、杀菌、杀虫、解冻等领域的潜力被广泛开发,并且在这些领域已有部分工业应用。文章综述了射频加热技术机理、在食品领域的应用以及射频加热过程的数学建模方法,指出了射频加热技术目前存在的加热不均匀性等问题及解决方案,并对未来研究的方向进行了展望。     

射频加热技术在农产品和食品加工中的应用

射频加热技术是近年来广受关注的一种加热方式，因其具有加热迅速、均匀性好、节约能源、维护成本低等特点，被广泛应用于食品和农产品的加工之中。该文首先对射频加热技术的工作原理和特点进行探讨，提出可以通过使用COMSOL软件的模拟实现来改善射频加热的均匀性；然后，通过分析近年来射频加热技术的应用文献，综述了射频加热技术在解冻、干燥、灭菌和杀虫等领域的研究进展；最后，提出了射频加热技术尚存的问题，并对未来研究方向进行展望。     

射频加热技术及其在果蔬干制中的研究应用进展

干制是最重要的果蔬加工技术之一,选择合适的干制技术与干燥能耗、产品品质密切相关。射频加热技术作为一种新型物理加热技术,因其加热迅速、具有体积加热效应、能量穿透深度大等优点,同时兼具微生物控制和灭酶效果,近年来备受关注。本文主要综述了射频加热技术的作用机理及特点,影响其在果蔬干制加工中应用的因素,归纳总结了射频加热技术在果蔬干制、微生物控制与灭酶中的应用现状,探讨了射频加热均匀性问题及可能的解决方案,并对未来研究方向进行了展望,为其今后在果蔬干制工业化应用提供参考。     

射频微波技术在食品与农产品灭菌杀虫中应用的研究进展

射频技术主要通过热效应对有害生物进行杀灭,具有效率高、加热均匀性好的优势。相比于微波在低含水量食品及农产品中加热效果更好。本文系统介绍谷物的射频灭菌杀虫机理、效果及应用,由于低频率电磁波用于加热处理时偶极弛豫对升温贡献所占比例降低而离子传导所占比例升高,以及低含水率食品中较低的热传导率,这些因素使得射频加热适合用于粮食等低含水量食品的灭菌杀虫操作。此外,射频频率电磁波长使得加热有较好的穿透性和内部加热均匀性。并且射频加热系统可以方便整合热风、真空、热循环等加热方式,从而达到提高加热效率、增强产品质量的目的。综上所述,射频加热相对于传统加热方式在加热效率上有很大优势,可明显降低粮食等低含水量食品由于有害生物造成的损失,对农产品保存有重大的意义。本文为粮食等低含水量农产品保存中应用射频处理技术提供依据及参考。     

射频解冻过程中食品加热均匀性影响因素的研究进展

射频加热(radio frequency, RF)技术作为一种新兴加热技术，是一种高效、低能耗的可替代传统加热的热处理方法。该方法在提高工业效率的同时又可保证食品的质量安全性及品质，可满足食品工业中的解冻以及其他加工需求，在食品工业中具有越来越大的应用潜力和发展前景。但解冻过程中存在的“边缘效应”等加热不均匀问题仍是制约该技术的一个瓶颈。本研究对射频解冻及其原理进行了介绍，并总结了影响解冻过程中加热均匀性等问题的因素，包括电磁场、射频功率、电极电压及间隙、食品的介电特性等，以期为射频解冻技术的研究及改善加热均匀性等未来研究发展方向提供参考。     

射频加热防治5种储粮害虫的初步研究

储粮害虫是造成粮食数量和质量损失的重要因素,为了寻求绿色、安全、生态的杀虫技术,本研究针对小麦中5种主要储粮害虫,使用频率为40.68 MHz的射频杀虫仪研究物理加热杀虫技术。针对粮库中常见的耐温害虫谷蠹,研究射频处理时间、极板间距及粮层厚度对射频杀虫的影响。筛选出射频杀虫的最优条件,在此条件下研究对赤拟谷盗、锯谷盗、长角扁谷盗和玉米象的杀灭效果,并初步分析射频处理对小麦成分的影响。结果表明,针对使用的射频杀虫仪,粮层厚度9 cm、极板间距10 cm、处理时间330 s为最优杀虫条件,在该条件下,其余4种主要害虫的死亡率均达到90%以上。射频处理对小麦中的水分、湿面筋值等成分不会造成显著变化,不影响小麦的食用价值。本研究为射频加热物理防治储粮害虫提供重要的基础参数,也为储粮害虫的绿色生态综合控制技术提供了新思路和新方法。     

微波加热在毛条烘干中的应用

介绍了应用微波加热烘干毛条的基本原理,研究了微波干燥设备的烘干效率、节能效果及烘干效果,结果表明,微波加热可以应用于纺织印染行业,为微波加热在纺织印染行业的推广应用提供了研究基础。通过生产应用,在计算微波加热毛条烘干工艺参数时,微波烘干机的干燥效率可取1.1 kg/(kW.h),微波干燥后,毛条含水率均匀,颜色前后色差无变化,纤维的强度和手感得到改善,微波干燥节能效果,比射频烘干节能约20%。     

苹果差压预冷均匀性的实验研究

通过搭建差压预冷实验台研究送风速度、送风温度、开孔大小以及开孔方式4个因素对差压预冷冷却均匀性的影响。利用混合正交试验分析法研究以上4个因素对冷却均匀性的影响程度。结果表明:送风速度越大、送风温度越高预冷均匀性越好,预冷过程中均匀度峰值最低;对于圆形开孔,孔径越大,均匀度越好;对于椭圆形开孔,孔径越小,均匀度越好;孔径大小不同时,均匀性变化过程相似,峰值出现所需预冷时间相同(峰值出现的位置相同);不同送风速度、送风温度、开孔大小和开孔方式,均匀度均呈先增大后减小的趋势;不同送风速度、送风温度、开孔大小和开孔方式对均匀度影响程度大小依次为送风温度、送风速度、开孔大小、开孔方式。     

食品射频灭菌过程加热均匀性调控研究进展

作为一种新型物理灭菌技术,射频(radio frequency, RF)加热具有穿透深度大、传热效率高、无化学残留、适合批量处理等特点,可有效杀灭食品中的致病菌。然而,受物料本身特性参数、射频系统结构、物料周围介质等因素影响,非均匀加热一直是射频灭菌规模化应用面临的主要挑战之一。为进一步明确食品射频灭菌过程均匀性调控的最新研究进展,本文基于射频杀菌的机制及特点,对调控射频加热均匀性的具体措施进行了梳理,包括调控物料介电参数、改变环境介质、优化射频加热系统等,为加热均匀性调控的进一步研究及射频灭菌技术的进一步应用奠定基础。     

含绝缘体物料的欧姆加热过程模拟

欧姆加热利用食品物料本身的电阻抗来产生热量,是一种内加热方式,与传统加热方式相比,具有快速均匀的加热特点。但食品往往不是一个均匀体系,不同部位的电导率的差异也会造成欧姆加热升温的不均匀性,研究不均匀体系欧姆加热时的温度分布对于欧姆加热技术的开发利用有重要意义。本文利用有限元方法,模拟了含有绝缘体的物料的欧姆加热过程,并用实验进行了验证。结果表明,有限元模型能够很好地模拟欧姆加热过程,整个加热过程中,模拟预测的温度与实验测得温度接近,最大差值为8K。同时运用有限元模型模拟了绝缘体的位置和方位对温度场的影响,绝缘体在物料中的位置和方位影响物料中电流密度的分布,温度场最高点位置始终出现在电流密度最高的区域,而最低点与电流的加热作用和物料间的热传导作用有关。     

Effect of dispersion and ion concentration on radio frequency heating

Radio frequency (RF) heating has been applied to process foods due to its unique advantages like volumetric heating. To investigate the interaction between dispersed liquid food and electromagnetic field, four dispersion structures, formed by polypropylene pellets dispersed in the samples, and six solutions with different ion concentrations were analyzed. The Results showed that 4 mm dispersion structure and 0.01 mol/L ion concentration involved in the highest heating rate and made the heating rate increase from 1.23 °C/min to 5.53 °C/min. For materials with different ion concentrations, the maximum heating rate corresponded to the dispersion structure of different sizes. But the dispersion structure would reduce the heating uniformity of the horizontal surface of materials. It suggested that dispersion structure and an proper ion concentration could change the material into a dispersed status, further improve RF heating rate, and ensure the efficiency of sterilization as well as retain the nutrition of foodstuffs.     

Sterilization of Foodstuffs Using Radio Frequency Heating

ABSTRACT: A study was conducted with a pilot-scale sterilization system based on 27-MHz radio frequency (RF) energy to investigate the effectiveness in shortening process time and in improving quality for foods sealed in 6-pound military-ration polymeric trays. Chemical marker M-1 was used to evaluate heating uniformity in 20% whey protein gels as a model food, and macaroni and cheese was processed to assess the influence of RF process on product quality. With the RF system, a lethality ( F ₀= 10 min) was achieved in both model food and macaroni and cheese within 30 min with relative uniform heating, compared to a 90 min conventional retort process that delivered a similar lethality.     

Evaluating performances of a small-scale 50 Ω radio frequency heating system designed for home applications

Industrial-scale radio frequency (RF) heating systems have been extensively used in processing agricultural products and foods. However, small-scale RF heating systems for household applications are yet to be realized. The purpose of this study was to develop small-scale 50 Ω RF heating systems and evaluate its heating performances with cooked rice (moisture content (MC) of 62.5 ± 0.5% wet basis (w.b.)). Two RF systems (13.56 and 27.12 MHz) were developed, and the heating rate, heating uniformity, and energy efficiency of cooked rice samples were compared between the developed RF units and a 2450 MHz microwave (MW) oven under static conditions. Results showed that the RF heating systems had smaller temperature differences and better heating uniformity than the MW oven under similar overall heating rate and the same input power (700 W) levels. The 13.56 MHz RF system performed better than the 27.12 MHz one. These findings may provide some clear performance indicators and guidelines for domestic application of small-scale 50 Ω RF heating systems.     

Developing radio frequency technology for postharvest insect control in milled rice

Since methyl bromide fumigation has an adverse effect on human health and environment, it is urgently needed for developing a non-chemical method to replace chemical fumigation for disinfesting milled rice. The purpose of this research was to study possible applications of radio frequency (RF) energy for disinfesting milled rice without affecting product quality. A pilot-scale, 27.12 MHz, 6 kW RF system was used to study RF heating uniformity and develop a treatment protocol for achieving 100% insect mortality and finally evaluating quality attributes in RF treated milled rice during storage. The results showed that the heating time needed only 4.3 min to heat the 3.9 kg milled rice from 25 °C to 50 °C using RF energy, but 480 min for milled rice to reach 48 °C using hot air at 50 °C. After comparing three selected electrode gaps, an appropriate gap of 11 cm was obtained to achieve the heating rate of 5.8 °C/min for further heating uniformity tests. An RF treatment protocol was finally developed to combine 1.0 kW RF power with a forced hot air heating at 50 °C, movement of the conveyor with the speed of 12.4 m/h, two mixings, and holding at 50 °C hot air for 5 min, followed by forced room air cooling through single-layer (2 cm thick) samples. There were no significant differences in quality parameters (moisture, protein, fat, starch, hardness, and color) between RF treatments and untreated controls during storage (P > 0.05). Therefore, RF treatments may provide a practical, effective and environmentally friendly method for disinfesting milled rice.     

加热方式对鱼糜凝胶特性的影响

以鲢鱼糜为原料,研究蒸制、煮制及微波加热对鱼糜凝胶品质的影响,采用分形分析鱼糜凝胶微观结构,并初步探讨鱼糜微观结构与鱼糜凝胶特性的相关性。结果表明：加热方式显著影响鱼糜凝胶的硬度、咀嚼性、凝胶强度、破断强度和微观结构参数(P≤0.05),对内聚性、凹陷深度、持水性有一定影响。相关性分析表明：分形维数较大和平均当量直径较小时,鱼糜凝胶的硬度、咀嚼性、凝胶强度、破断强度、凹陷深度和持水性都较大。     

Industrial-scale radio frequency treatments to control Sitophilus oryzae in rough,brown, and milled rice

Many previously studies have confirmed that radio frequency (RF) treatments have the potential to be developed as non-chemical alternative disinfestation methods. However, most of them are conducted in laboratory scale RF systems, and it is necessary to scale up the treatment protocol for industry scale applications. A pilot-scale, 27.12 MHz, 6 kW RF system was used to simulate the continuous industrial processing and finally estimate the heating efficiency and throughput for controlling adult rice weevils, Sitophilus oryzae (L.), in milled, brown, and rough rice. An electrode gap (11.5 cm) was chosen based on the appropriate heating rate (6–8 °C/min). RF heating uniformity in the three types of rice samples was improved by adding 50 °C forced hot air surface heating, sample movements on the conveyor belt at a speed of 12.5 m/h, and holding in 50 °C hot air for 6 min. The final industrial RF treatment achieved a complete mortality of adult S. oryzae and provided acceptable quality attributes in moisture content, water activity, color, protein, free fatty acid, and ash. The average heating efficiency and throughput of the RF treatments were 77.7, 76.3, and 74.3%, and 268.8, 247.3, and 224.8 kg/h for milled, brown, and rough rice, respectively. The industrial scale-up studies provide alternative physical methods for disinfesting milled, brown, and rough rice to replace chemical fumigation.     

Investigation of the Feasibility of Radio Frequency Energy for Controlling Insects in Milled Rice

Many studies showed that radio frequency (RF) holds great potential to control insects in grains, while a few more technical issues need to be addressed to further make this technology ready for industrial application. Therefore, the effect of RF heating rate on rice fissure ratio and broken rate, RF heating lethality on immature rice weevils, and RF disinfestation treatment on cooking and eating quality of milled rice was investigated in this study. Results indicated that RF heating rate had no significant influence on rice broken rate, while fast RF heating rate (>7.2 °C/min) had adverse effects on rice fissure ratio, and mild RF heating rate (<5.8 °C/min) had no significant influence on fissure ratio of milled rice. RF treatment (50 °C, 5 min) could completely control immature rice weevils in rough, brown, and milled rice. Even though RF disinfestation treatment (50 °C, 5 min) influenced the cooking quality (water absorption, adhesive strength) of milled rice, it had no significant effect on overall sensory quality. This study provided valuable information for considering the application of RF energy in milled rice process to control insects.     

Developing a rotation device in radio frequency systems for improving the heating uniformity in granular foods

A major obstacle for industrial radio frequency (RF) treatments is non-uniform and thermal runaway heating. This study aimed to improve the RF heating uniformity in granular products using a rotation device with sample mixing. After the run-out error of the rotation device met the allowable tolerance range, the RF heating uniformity was evaluated for mung bean, coix seed, peanut kernel and almond kernel under different rotating speeds, volume ratios and moisture contents (MCs). The results showed that the RF heating uniformity index decreased and then increased with increasing rotation speed, whereas the volume ratio presented the opposite effect. The RF heating uniformity index raised with increasing MCs. The heating uniformity in mung beans was the best because of their smallest particle size among the four selected samples. Therefore, the laboratory-scale rotation device applied in the RF system could provide a solid foundation for further large-scale industrial applications.     

Thermal behavior of CMC solutions under simulation of radio frequency pasteurization

Pasteurization and enzyme-inactivation are an economical and convenient way to provide safety and shelf-life stability. However, long heating time and non-uniform temperature distribution are the main issues of traditional thermal processing for enzyme-inactivation. Because of the volumetric heating and high heating rate, radio frequency (RF) treatment is considered as one of the most promising physical enzyme-inactivating methods. A 6 kW, 27.12 MHz RF heating system was applied to explore the effect of cylindrical container dimension and solution concentration on the thermal behavior of high-viscosity liquid food. Carboxymethylcellulose (CMC) solution as a non-Newtonian fluid was selected as a model liquid food in this study. The target temperature for enzyme inactivation was 70 °C. The experimental results showed that the electrode gap, container length, and container diameter affected the heating rate and uniformity of the liquid food. Furthermore, when the concentration of CMC solution increased from 0.5% to 2.0%, the heating rate of the solution firstly increased and then decreased, while the heating uniformity index of the solution increased from 0.008 ± 0.001 to 0.056 ± 0.002. The simulation results showed that the higher electric field strength led to a higher heating rate, temperature, and velocity of natural convection of the solution at the ends of the container. Then, the recirculation zones from the ends to the center of the container were formed, which was diminished with the increasing concentration solution, resulting in the increased maximum temperature difference of solution. The results of this research may provide useful data for subsequent continuous flow studies and information on potential RF enzyme-inactivation in high-viscosity liquid foods.